Transcript
TECHNIC AL
Modern Technology
Graeme Morpeth
Deep Breathing
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Supercharging & turbocharging Mercedes-Benz engines
ust say “SSK” to a Mercedes-Benz enthusiast and images of Carlos Antonio Zatuszek and Rudolf Caracciola racing along in European roads in Grands Prix, accompanied by the banshee wail of powerful, supercharged 420-cubic-inch straight-6 engines spring to mind. Engaging the supercharger increased horsepower from approximately 200 horsepower to 300 horsepower “at the flex of an ankle,” according to advertisements of the day. The famous Silver Arrows Grand Prix racing cars of the 1930s were also supercharged; the zenith, reached by the W125, powered by a twin-centrifugal supercharged 336-cubic-inch straight-8, producing 645 horsepower in a chassis that weighed 1,650 pounds – without the paintwork. Spring forward some 84 years and you will again find Mercedes-Benz Grand Prix cars, turbocharged this time, dominating the F1 scene. So what? How does any of this relate to the Mercedes in your garage? The progress that Mercedes-Benz has made in using these technologies in its road cars and the relative merits of the two forcedinduction systems is what is important here – supercharging versus turbocharging and the differences they can make to an engine in terms of power output. First, though, let’s establish a baseline. In ordinary engines, which consume air at atmospheric pressure, the gasoline combustion process will typically produce 1.0-1.34 horsepower per cubic inch, though highly tuned engines will yield up to 2.16 horsepower per cubic inch. Manufacturers have typically overcome this limitation by using larger engines to provide more power, a strategy nicknamed “no replacement for displacement.” The engines used for the U.S. version of the W202 C-Class sedan illustrate the point:
C230: M111 engine, 140 cubic inches, straight-4 rated at 148 horsepower and 162 pound-feet of torque C240: M112 engine, 156 cubic inches, V-6 rated at 168 horsepower and 177 pound-feet of torque C280: M104 engine, 170.8 cubic inches, straight-6 rated at 195 horsepower and 199 pound-feet of torque C36 AMG: M104 engine, 220 cubic inches, straight-6 rated at 280 horsepower and 284 pound-feet of torque
In comparison, the W203 C32 AMG was fitted with a supercharged M112 192 cubic-inch V-6 rated at 349 horsepower and 332 horsepower. Yikes! Compared with the naturally aspirated version of this engine, supercharging produced a 62 percent increase of power and a 45 percent increase in torque. So there is a replacement for displacement – forced induction! The term “forced induction” refers to the principle that if a greater volume of air can be forced into the engine than would occur naturally with the suction created by the piston receding in the cylinder, more fuel can be used and greater horsepower can be produced. There are two technologies that can produce this effect – superchargers and turbochargers. Both technologies use some form of air compression, with rotating cylinders, spinning blades or a screw mechanism pushing air into the engine. The difference between the two technologies is where the power comes from to spin the compressor. In a supercharger, a belt connected with the driveshaft spins the compressor; in a turbocharger, exhaust gas forced out of the cylinders spins a parallel compression mechanism directly connected by a shaft to the turbo-charging compressor. Each technology has its advantages and disadvantages. With power provided by a simple belt connection with the rotating driveshaft, a supercharger is easy to install and its effect is immediate. However, when it’s activated, it draws power parasitically from the engine, monopolizing as much as 40 percent of the available engine power. In direct contrast, the turbocharger uses available force from exhaust gases so it doesn’t draw power from engine combustion; but because the engine has to be at speed, the turbocharger takes a little time to spool up. Taking a look first at superchargers, there are three main types: the Roots Blower; the Lysholm Twin-Screw, and the Centrifugal Blower, as shown in Figure 1. The Roots Blower is probably the best-known type of supercharger and it is likely that Gottlieb Daimler was the first to apply this technology to any sort of 4-stroke internal combustion engine just before his death in 1900. The world’s first supercharged production cars using Roots Blowers were the Mercedes-Benz 6/25/40 and 10/40/65, both of which were introduced during the 1921 Berlin Motor Show. These cars, or variations of them, eventually became the fabulous SSK, SSKL and 540K models. A two-stage centrifugal supercharger was used on the 1938 W154 GP racing car to extract 476 horsepower from the M154 180-cubic-inch V-12 engine.
Figure 1: Superchargers ROOTS TRIPLE LOBE 60 HELIX O
Intake Side
LYSHOLM TWIN SCREW SUPERCHARGER
CENTRIFUGAL SUPERCHARGER
Intake Side Discharge Side Male Rotor
Female Rotor
Discharge Side Discharge Side 58
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Intake Side
TECHNIC AL A mighty and evocative supercharged 420-cubic-inch straight-6 engine powered the highly successful Mercedes-Benz SSK between 1928 and 1932. The efficient though complex Lysholm supercharger was used in the 2001 C32 AMG and increased the output of the M112 E32 192-cubicinch V-6 engine from 224 to 354 horsepower and the pound-feet of torque from 232 to 332. The rotors were Teflon coated, spinning at more than 20,000 rpm. Because the action of compressing air creates heat, a water-to-air intercooler was part of the package. MercedesBenz last used supercharger technology in 2006 in the CLK and SLK (with K short for Kompressor). These days, Mercedes-Benz and AMG use a combination of turbocharged and naturally aspirated engines. Turbochargers offer an alternative route to “replacing displacement,” though the method of operation and installation is more complicated. Figure 2 shows the two basic elements of a turbocharger: the turbine into which exhaust gasses (red) are fed and then discharged through the exhaust system. The compressor on the other side draws ambient temperature air (blue) and discharges it into the inlet manifold of the engine. The compressor-outlet pressure, normally called the boost (pressure), typically varies between 15-30 psi for run-of-the-mill engines and up to 60 psi (or more) for highly tuned racing engines. The first production turbocharged diesel passenger car was the 1978 W116 300SD, which used a 5-cylinder 180-cubic-inch OM617 engine; in normally aspirated form, it produced 80 horsepower and 122 pound-feet of torque; in its final turbocharged form, it produced 122 horsepower and 170 pound-feet.
For turbocharger installation, difficulties come in many forms: ◆◆
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Air discharged from the compressor is hot because it has been compressed The turbine and compressor casings are subject to heat soak from exhaust gases passing through the turbine The hot air needs to be cooled before being ingested by the engine, so intercoolers, either air-air or air-water, are required The plumbing required to install a turbocharger is complicated, including intercoolers, revised inlet and exhaust systems, safety valves and the like Both the turbine and compressor are fitted with safety valves to prevent over-pressurization The time needed for the turbo to spool up and start producing boost can affect the drivability of the car
It seems like a lot of effort to get more power and torque from an engine, but there is more to it than that. A great example of a modern diesel engine, the 4-cylinder OM651 as used in the 2014 E250, shown in Figure 3 on page 60. The twin-sequential variable-vane turbo system used on this engine is complex, but provides considerable benefits. Why twin turbochargers? The smaller one (A) of the two spools up faster at low revs, thus reducing turbo lag to a minimum. The large
Figure 2: The Turbocharger
COMPRESSOR SIDE Ambient air (blue) is drawn in and then discharged into the engine inlet manifold
TURBINE SIDE Exhaust gasses (red) fed through turbine and then out exhaust system
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C C
TECHNIC AL
C
A Small Turbocharger A
A
B Large Turbocharger C Exhaust Gas Recirculation System
B B
Figure 3: The OM651 diesel engine with twin-sequential variable-vane turbo system turbo (B) takes longer to spool up and thus provides mid-range and top-end boost. The installation is complicated by the Exhaust-Gas Recirculation system (C), which takes up to 30 percent of the exhaust gas, cools it and then feeds it back into the engine. The objective here is to reduce emissions and, paradoxically, reducing the temperature of the combustion process reduces the amount of nitrous oxides (NOx) and other pollutants produced. Mercedes-Benz uses the BlueTEC system to further reduce this pollution. It works by injecting a urea-water mix into the exhaust catalyst where urea form ammonia, which reacts with the NOx to form water and nitrogen. This 126-cubic-inch 4-cylinder engine has astonishing power output: 195 horsepower at 3,800 rpm and 369 pound-feet of torque at 1,600-1,800 rpm: Compare this with the M276 V-6 210-cubic-inch gasoline engine, which produces 302 horsepower and (only) 273 pound-feet. Fuel economy is outstanding for this engine and the driver of an E250 BlueTEC can expect to breach the 40 mpg barrier. This engine is considered to be so powerful and reliable that M-B now uses a slightly detuned version of it in Sprinter vans as the primary engine; the option is the OM642 turbo-charged V-6, rated at 188 horsepower and 325 pound-feet of torque. However, it’s not just diesel engines that Mercedes turbocharges these days. For instance, the 2014 R231 SL550 comes with the M278 282-cubic-inch twin-turbocharged gasoline V-8 that produces 429 horsepower and 516 pound-feet of torque. In this engine, the turbochargers form an integral part of the exhaust manifold. The
photographs show the barrel exhaust manifold, designed to reduce turbo lag, the wastegate and control diaphragm, the catalytic converter and the two Lambda sensors. The SL63 and SL65 AMG versions have significantly more power and torque, as might be imagined. So where has all this taken us? In general terms, superchargers have proven to be simple and cheap to install but problematical in operation because of the parasitic load required to power them. Turbochargers, on the other hand, are expensive and require sophisticated control systems and cooling systems but are more efficient in their operation. Witness the performance of the 2014 Mercedes AMG Petronas F1 cars, which won 16 of 19 races during that season. They are powered by extremely sophisticated turbocharged, 90-cubic-inch V-6 engines rated at over 600 horsepower, with the turbochargers driven not only by exhaust gases, but also during off-boost moments by an energy-efficient electric motor – clever stuff indeed. The installation overcomes many of the problems noted above by locating the compressor and turbine at opposite ends of the engine block, thus reducing heat transfer to a minimum. These engines and their associated hybrid power-packs are so efficient that they can complete at least 190 miles on less than 27.5 gallons of fuel, a considerable savings over the old naturally aspirated engines; it is this technology that M-B is transferring to its road vehicles. Given the increased horsepower from smaller, more fuel-efficient engines with reduced emissions that can be achieved using turbocharged forced-induction technology, we can expect to see increased applications in future Mercedes-Benz cars.
LEFT: The barrel exhaust manifold, designed to reduce turbo lag. RIGHT: The wastegate and control diaphragm, the catalyzer and the two Lambda sensors. 60
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